Inputs

The operator to be defined. See below for allowable
characters. The name may be schema-qualified, for example
CREATE OPERATOR myschema.+
(...).

func_name

The function used to implement this operator.

lefttype

The type of the left-hand argument of the operator, if
any. This option would be omitted for a left-unary
operator.

righttype

The type of the right-hand argument of the operator,
if any. This option would be omitted for a right-unary
operator.

com_op

The commutator of this operator.

neg_op

The negator of this operator.

res_proc

The restriction selectivity estimator function for
this operator.

join_proc

The join selectivity estimator function for this
operator.

HASHES

Indicates this operator can support a hash join.

MERGES

Indicates this operator can support a merge join.

left_sort_op

If this operator can support a merge join, the
less-than operator that sorts the left-hand data type of
this operator.

right_sort_op

If this operator can support a merge join, the
less-than operator that sorts the right-hand data type of
this operator.

less_than_op

If this operator can support a merge join, the
less-than operator that compares the input data types of
this operator.

greater_than_op

If this operator can support a merge join, the
greater-than operator that compares the input data types
of this operator.

Outputs

CREATE
OPERATOR

Message returned if the operator is successfully
created.

Description

CREATE OPERATOR defines a new
operator, name. The user who
defines an operator becomes its owner.

If a schema name is given then the operator is created in the
specified schema. Otherwise it is created in the current schema
(the one at the front of the search path; see CURRENT_SCHEMA()).

Two operators in the same schema can have the same name if
they operate on different data types. This is called overloading. The system will attempt to pick the
intended operator based on the actual input data types when there
is ambiguity.

The operator name is a
sequence of up to NAMEDATALEN-1 (63 by
default) characters from the following list:

+ - * / < > = ~ ! @ # % ^ & | ` ? $

There are a few restrictions on your choice
of name:

$ cannot be defined as a
single-character operator, although it can be part of a
multicharacter operator name.

-- and /* cannot appear anywhere in an operator name,
since they will be taken as the start of a comment.

A multicharacter operator name cannot end in + or -, unless the
name also contains at least one of these characters:

~ ! @ # % ^ & | ` ? $

For example, @- is an allowed operator name, but *- is not. This restriction allows
PostgreSQL to parse
SQL-compliant queries without requiring spaces between
tokens.

Note: When working with non-SQL-standard operator
names, you will usually need to separate adjacent operators
with spaces to avoid ambiguity. For example, if you have
defined a left-unary operator named @, you cannot write X*@Y; you must write X*
@Y to ensure that PostgreSQL reads it as two operator
names not one.

The operator != is mapped to
<> on input, so these two names
are always equivalent.

At least one of LEFTARG and
RIGHTARG must be defined. For binary
operators, both should be defined. For right unary operators,
only LEFTARG should be defined, while
for left unary operators only RIGHTARG
should be defined.

The func_name procedure must
have been previously defined using CREATE
FUNCTION and must be defined to accept the correct number of
arguments (either one or two) of the indicated types.

The commutator operator should be identified if one exists, so
that PostgreSQL can reverse the
order of the operands if it wishes. For example, the operator
area-less-than, <<<, would probably have a commutator
operator, area-greater-than, >>>. Hence, the query
optimizer could freely convert:

box '((0,0), (1,1))' >>> MYBOXES.description

to

MYBOXES.description <<< box '((0,0), (1,1))'

This allows the execution code to always use the latter
representation and simplifies the query optimizer somewhat.

Similarly, if there is a negator operator then it should be
identified. Suppose that an operator, area-equal, ===, exists, as
well as an area not equal, !==. The negator link allows the query
optimizer to simplify

NOT MYBOXES.description === box '((0,0), (1,1))'

to

MYBOXES.description !== box '((0,0), (1,1))'

If a commutator operator name is supplied, PostgreSQL searches for it in the catalog.
If it is found and it does not yet have a commutator itself, then
the commutator's entry is updated to have the newly created
operator as its commutator. This applies to the negator, as well.
This is to allow the definition of two operators that are the
commutators or the negators of each other. The first operator
should be defined without a commutator or negator (as
appropriate). When the second operator is defined, name the first
as the commutator or negator. The first will be updated as a side
effect. (As of PostgreSQL6.5, it also works to just have both
operators refer to each other.)

The HASHES, MERGES, SORT1, SORT2, LTCMP, and
GTCMP options are present to support the
query optimizer in performing joins. PostgreSQL can always evaluate a join (i.e.,
processing a clause with two tuple variables separated by an
operator that returns a boolean) by
iterative substitution . In addition, PostgreSQL can use a hash-join algorithm ;
however, it must know whether this strategy is applicable. The
current hash-join algorithm is only correct for operators that
represent equality tests; furthermore, equality of the data type
must mean bitwise equality of the representation of the type.
(For example, a data type that contains unused bits that don't
matter for equality tests could not be hash-joined.) The
HASHES flag indicates to the query
optimizer that a hash join may safely be used with this
operator.

Similarly, the MERGES flag indicates
whether merge-sort is a usable join strategy for this operator. A
merge join requires that the two input data types have consistent
orderings, and that the merge-join operator behave like equality
with respect to that ordering. For example, it is possible to
merge-join equality between an integer and a float variable by
sorting both inputs in ordinary numeric order. Execution of a
merge join requires that the system be able to identify four
operators related to the merge-join equality operator: less-than
comparison for the left input data type, less-than comparison for
the right input data type, less-than comparison between the two
data types, and greater-than comparison between the two data
types. It is possible to specify these by name, as the SORT1, SORT2, LTCMP, and GTCMP options
respectively. The system will fill in the default names
<, <,
<, >
respectively if any of these are omitted when MERGES is specified. Also, MERGES will be assumed to be implied if any of
these four operator options appear.

If other join strategies are found to be practical,
PostgreSQL will change the
optimizer and run-time system to use them and will require
additional specification when an operator is defined.
Fortunately, the research community invents new join strategies
infrequently, and the added generality of user-defined join
strategies was not felt to be worth the complexity involved.

The RESTRICT and JOIN options assist the query optimizer in
estimating result sizes. If a clause of the form:

myboxes.description <<< box '((0,0), (1,1))'

is present in the qualification, then PostgreSQL may have to estimate the fraction
of the instances in myboxes that satisfy
the clause. The function res_proc
must be a registered function (meaning it is already defined
using CREATE FUNCTION) which accepts
arguments of the correct data types and returns a floating-point
number. The query optimizer simply calls this function, passing
the parameter ((0,0), (1,1)) and
multiplies the result by the relation size to get the expected
number of instances.

Similarly, when the operands of the operator both contain
instance variables, the query optimizer must estimate the size of
the resulting join. The function join_proc will return another floating-point
number which will be multiplied by the cardinalities of the two
tables involved to compute the expected result size.

The difference between the function

my_procedure_1 (MYBOXES.description, box '((0,0), (1,1))')

and the operator

MYBOXES.description === box '((0,0), (1,1))'

is that PostgreSQL attempts
to optimize operators and can decide to use an index to restrict
the search space when operators are involved. However, there is
no attempt to optimize functions, and they are performed by brute
force. Moreover, functions can have any number of arguments while
operators are restricted to one or two.

Notes

Refer to the chapter on operators in the PostgreSQL User's Guide for further
information. Refer to DROP OPERATOR to
delete user-defined operators from a database.

To give a schema-qualified operator name in com_op or the other optional arguments,
use the OPERATOR() syntax, for
example

COMMUTATOR = OPERATOR(myschema.===) ,

Usage

The following command defines a new operator, area-equality,
for the BOX data type: